System and process for the production of UAN

This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Patent Application No. PCT/NL2024/050422, filed Jul. 29, 2024, which claims priority to EP 23188198.8, filed Jul. 27, 2023, which are incorporated by reference in their entireties.

The invention pertains to the production of a solution of urea ammonium nitrate in water (UAN). Particularly, the invention is directed to a system and method for processing an ammonium nitrate (AN) waste stream into UAN.

Urea ammonium nitrate (UAN) is an aqueous solution of urea and ammonium nitrate and is used as fertilizer. A process for producing UAN generally comprises providing an aqueous solution of ammonium nitrate (AN), providing an aqueous solution of urea, and mixing the ammonium nitrate and urea solutions in a UAN production unit, so as to produce urea ammonium nitrate. The ammonium nitrate and the urea can be provided, e.g., by production in separate ammonium nitrate and urea plants, or in corresponding sections of combined or integrated plants. Also, it has been proposed that the ammonium nitrate is provided in part as a byproduct of an acidic ammonia scrubbing unit.

Background art related to sending all or part of a scrubbing liquid used in an acidic ammonia scrubber to a UAN production unit, is known from WO2017/111588 or from WO2017/111585. The set-up disclosed herein, is limited to an integration with urea production. In both documents, additional ammonium nitrate from a separate ammonium nitrate production section is supplied to the UAN production unit. It would be desired to provide a system for the production of UAN that can be applied to largely any ammonium nitrate waste stream. A further desire thereby is to decrease the discharge of emissions, especially ammonia, nitric acid, and ammonium nitrate, and to minimize waste streams and equipment costs. Also, it would be desired to further improve the efficiency of a system such as disclosed in said background art.

The invention also address the handling of acid aqueous ammonium nitrate solutions that result from acidic scrubbing of ammonia-containing off-gas, e.g. from the acid scrubbing of off-gas (waste air stream) from a finishing section of a urea plant.

Various urea production plants and processes are illustrated in Ullmann's Encyclopaedia of Industrial Chemistry, Chapter Urea, 2010. In many urea production processes, the formed urea melt is at least in part subjected to a finishing step where it is transformed into a solid urea product using a cooling gas stream, e.g. cooling air stream, for instance by granulation or prilling. The resulting off gas, i.e. waste air stream, comprises urea dust and NH, and air. The urea dust and ammonia can be removed from the waste air stream by scrubbing. Many processes use combined urea dust scrubbing and acid scrubbing, giving an aqueous solution comprising urea and ammonium salt. It has been proposed to use such a solution as a liquid fertilizer.

In order to better address one or more of the aforementioned desires, the invention provides, in one aspect, a system for the production of urea ammonium nitrate (UAN) comprising an inlet for aqueous ammonium nitrate, an inlet for liquid urea, and an outlet for UAN, the system comprising a concentration section configured to subject aqueous ammonium nitrate to evaporation so as to provide concentrated ammonium nitrate and, downstream of the concentration section and in fluid communication therewith, a production section configured to mix concentrated ammonium nitrate and liquid urea so as to produce UAN, wherein the concentration section has a gas outlet for water vapor, said gas outlet preferably being in fluid communication with a treatment section configured to subject water vapor received from the concentration section to scrubbing, said treatment section having an inlet for scrubbing liquid and an outlet for used scrubbing liquid, wherein the outlet for used scrubbing liquid is preferably in fluid communication with an inlet of the concentration section, and wherein the system comprises a pH control section such that the inlet for aqueous ammonium nitrate is in fluid communication with an inlet of the pH control section, wherein the pH control section is configured to subject aqueous ammonium nitrate to pH control, so as to provide a pH-controlled aqueous ammonium nitrate stream, wherein the pH control section has an outlet for the pH-controlled aqueous ammonium nitrate stream in fluid communication with an inlet of the concentration section, in particular the evaporation section.

In another aspect, a process is provided for the production of urea ammonium nitrate (UAN), comprising obtaining a used scrubbing liquid resulting from subjecting an ammonia-containing off-gas to contact with nitric acid in a scrubber; subjecting the used scrubbing liquid to pH control, said pH control comprising determining pH and adjusting pH to the extent necessary so as to be within a range of 2.0 to 4.5 so as to provide a pH-controlled liquid, subjecting said pH-controlled liquid to evaporation so as to obtain a concentrated ammonium nitrate solution and combining said concentrated ammonium nitrate solution and an aqueous urea solution and/or a urea melt so as to obtain a urea ammonium nitrate solution. The process is advantageously, but not exclusively, carried out in the system of the invention.

In a still further aspect, a method is presented for modifying a pre-existing chemical processing unit having an outlet for a waste aqueous ammonium nitrate stream, the method comprising providing a system for the production of urea ammonium nitrate (UAN) as described above, and connecting said system to said chemical processing unit such that the outlet for the waste aqueous ammonium nitrate of the chemical processing unit is in fluid communication with the inlet for aqueous ammonium nitrate of the system for the production of UAN.

Urea Ammonium Nitrate (UAN) is a fertilizer which is generally used as an aqueous solution of urea and ammonium nitrate. Ammonium nitrate is produced by reacting ammonia with a strong solution of nitric acid while maintaining the pH of the solution within narrow boundaries. The resulting solution is then mixed with an aqueous urea solution to obtain UAN. Typical UAN products contain 28 wt. % to 32 wt. % of total nitrogen. The UAN product typically comprises of from 29 wt. % to 38 wt. % urea and of from 36 wt. % to 48 wt. % of ammonium nitrate, with the remainder being water. Preferably the UAN contains max. 35 wt. % water, preferably 20-30 wt. % water. The inventive UAN production process preferably yields such UAN. Hence, the water content of the UAN preferably is not too high. This is especially advantageous for UAN that is used as fertilizer. The invention is based on the judicious insight to conduct the production of UAN on the basis of an acid aqueous ammonium nitrate stream, which is a used scrubbing liquid, that is subjected to pH control, prior to subjecting it to evaporation and mixing it with urea. Preferably, this is done in combination with a treatment step, based on scrubbing, of the vapor from evaporation, preferably with a recirculation step of the liquid from the treatment.

The system disclosed herein, can be put to use in connection with, and can be a part of, any plant from which an acid liquid stream comprising ammonium nitrate is obtained as a by-product or waste stream. Particularly this refers to any such streams that result from subjecting an ammonia-containing off-gas to contact with nitric acid (NA) in a scrubber. The acid aqueous ammonium nitrate stream concerned is thus a used scrubbing liquid, and the stream can also be named ‘used scrubbing liquid’. With reference to the fact that the off-gas is a by-product of another chemical reaction, it will be understood that the acid aqueous ammonium nitrate is typically a small waste stream. The present invention seeks particularly to provide a system that is designed for processing such small ammonium nitrate waste streams to UAN. Typically, and in example embodiments of the invention, such small waste streams are in a range of from 1 to 4 Ton/h, such as 2 Ton/h, comprising 5-15 wt. % of ammonium nitrate, typically 10 wt. %, a small amount of nitric acid, typically 0.3-1.0 wt. %, the remainder being water. In an example embodiment, in the event of a urea plant producing a urea melt, the AN waste stream is typically less than 5 wt. %, more typically less than 3 wt. % of the total urea melt produced by such plant. In an embodiment of the process and of the system, the aqueous ammonium nitrate stream comprises 5-15 wt. % of ammonium nitrate, typically 10 wt. %, a small amount of nitric acid, typically 0.3-1.0 wt. %, the remainder being water, and max. 5 wt. % or max 1.0 wt. % urea; this typical composition applying independent of the flow rate. The low level of urea, e.g. only traces of urea, applies in particular if dust scrubbing is used upstream of the acid scrubbing.

This system can be integrated with a chemical processing unit producing an aqueous ammonium nitrate waste stream, typically originating from an acidic ammonia scrubber, such as ammonia plants, ammonium nitrate plants, manure-processing units, composting units, waste processing plants, coke manufacturing plants, urea melt/granulation plants. I.e., the system can be built in into a grass-roots plant comprising an acidic ammonia scrubber. The system can also be built in into a pre-existing plant. Still more advantageously, the system can be provided as a separate unit, that can be connected with any plant for which a useful application is sought for a by-product or waste stream resulting from subjecting an ammonia-containing off-gas to acidic scrubbing.

Such versatile application of the system of the invention is secured by the presence of a judiciously positioned pH control section. This pH control section is positioned such that the inlet for aqueous ammonium nitrate, is in fluid communication with the inlet of the pH control section. I.e., irrespective of whether any further sections, valves, pumps, or other types of equipment are present between the inlet for aqueous ammonium nitrate into the system, the pH-control section is downstream of such inlet and upstream of the evaporation section. The latter is secured with reference to the pH control section having an outlet for the pH-controlled aqueous ammonium nitrate stream in fluid communication with an inlet of the evaporation section.

The pH control section is configured to determine and/or adjust the pH of a stream subjected to such control. Determining pH can involve a pH measurement, or a calculation based on process parameters. Depending on the origin of the acid aqueous ammonium nitrate stream, such calculation can be made in advance, or can be made in situ, such as by a data processing unit receiving input of relevant data related to the scrubbing process from which the acid aqueous ammonium nitrate stream (i.e. the first used scrubbing liquid in) originates. An actual pH measurement of the acid aqueous ammonium nitrate stream can be conducted anywhere in a tract from within the scrubber up to and including inside of the pH control unit. Said pH control secures that the liquid subjected to such control, downstream of the pH control section, has a controlled pH within a desired range, or of a desired value. It will be understood that pH control does not necessarily involve an adjustment of the pH, in the event that the pH of the stream is determined to already have the desired value. In the pH control section, the pH of the aqueous ammonium nitrate liquid is generally controlled at a value in a range of from weak acidic to neutral, generally a pH of from 2.0 to 7.0.

Preferably, with the advantage of preventing the presence of a possible excess of ammonia, the pH is kept at a slightly acidic value, preferably in a range of 2.0 to 4.5. more preferably 2.5 to 3.5, such as at 3.0. This preference applies to the system and to the process. In this way, a high UAN yield is obtained with low NHemissions from the downstream concentrating of the pH-controlled liquid to concentrated AN solution.

The system comprises a concentration section, that is preferably an evaporation section, more preferably is provided by a heater, e.g. a heat exchanger. For instance evaporation by heating with heat exchange with steam is used.

The concentration section is configured to subject an aqueous ammonium nitrate stream, from the pH control section, which is a liquid stream, to evaporation so as to provide concentrated ammonium nitrate, and water vapors, i.e. a separate vapor stream comprising water vapor and entrained ammonium nitrate and nitric acid. The concentration section has an inlet for the pH-controlled aqueous ammonium nitrate, an outlet for the concentrated ammonium nitrate, and an outlet for the water vapors.

The system of the invention comprises a treatment section, which is optional for the process. The treatment section allows recovering and recirculating nitrogen compounds that become entrained in the vapor that results from concentrating the pH-controlled aqueous ammonium nitrate stream by evaporation. This feature serves to prevent unwanted emissions of nitrogen compounds, particularly originating from nitric acid and ammonia entrainments, into the air, in an embodiment wherein the concentration section has a fluid communication with an outlet, such as a stack, for water vapor, e.g, wherein the cleaned vapor from the treatment section is vented.

The vapor typically contains entrained ammonium nitrate (e.g. in the range 0.05-0.2 wt. % AN) and nitric acid (e.g. 0.05-0.2 wt. % NA); with nitric acid typically being gaseous (vapor).

The treatment section is also useful in an embodiment wherein the water vapors after the treatment are condensed and at least a portion of the condensate thereby obtained can be recirculated or otherwise transferred within a plant, rather than being directly vented. Advantageously, such recirculation can be to the treatment section as a treatment liquid for scrubbing off nitric acid and ammonium nitrate entrainments. It will be understood that also with a view to condensation and recirculation, it will generally be undesirable to have nitrogen compounds present in the vapor.

The treatment section is configured to subject the water vapor received from the concentration section (e.g. evaporation section) to scrubbing with a treatment liquid, also called scrubbing liquid (first scrubbing liquid in). To this end, the treatment section has an inlet for treatment liquid (scrubbing liquid), and an outlet for used treatment liquid. It will be understood that the treatment liquid will typically be an aqueous liquid stream, e.g. water, e.g. provided as condensate, for instance as steam condensate, e.g. with pH of at least 6, for instance pH 6 to 8. The amount of treatment liquid is e.g. at least 10 wt. % of the acid aqueous ammonium nitrate stream, e.g. in the range 10-30 wt. %.

The treatment section advantageously enables recirculating recovered nitrogen (notably ammonium nitrate and nitric acid) to the UAN production process. To this end, the treatment section has preferably an outlet for used treatment liquid that is in fluid communication with an inlet of the concentration section. Accordingly, recovered nitrogen will be added, as an aqueous used scrubbing liquid, preferably to the pH-controlled ammonium nitrate stream upstream of the concentration section. The treatment section can be referred to as a scrubber, and is e.g. a trayed column scrubber; other types of scrubbers are also possible.

The treatment section is distinct from and separate to any acid scrubber that is used for scrubbing off-gas (waste air) from a urea plant finishing sections giving in a preferred embodiment the acid aqueous ammonium nitrate stream; and the compositions of the gas streams are different. Furthermore, preferably the treatment section uses a treatment liquid that is not acid, preferably has a pH in the range 6 to 8.

The treatment section also has an outlet for cleaned vapor. The system may comprise a condenser connected to receive said cleaned vapor. The condenser comprises an outlet for uncondensed gas and an outlet for condensate. The system may comprise a liquid flow line to supply said condensate at least in part to an inlet of the treatment section.

The presence of the pH control section, and particularly the judicious positioning thereof upstream of the concentration section, provides, as one advantage, a beneficial synergistic effect with the presence of the treatment section, and particularly the recirculation of used treatment liquid therefrom in a preferred embodiment to an inlet of the concentration section. This recirculation would normally result in acid build-up within either or both of the treatment section and the evaporation section. By allowing pH-control of the liquid to be evaporated upstream of the concentration section, at any point in time the presence of acid can be counteracted by allowing the recirculated treatment liquid to be taken up into a less acidic aqueous ammonium nitrate stream.

The pH adjustment also advantageously avoids a decrease of the thermal stability of the ammonium nitrate, thus preventing the ammonium nitrate from becoming prone to decomposition at lower temperatures, which evidently adds to the safety of the process. The pH adjustment, in particular pH increase to less acidic pH, provides the advantage of a reduced risk of AN decomposition in the heating of the concentration section.

This advantage of the pH control step is obtained both in embodiments wherein the used treatment liquid is supplied to the inlet of the concentration section and in embodiments wherein the used treatment liquid is supplied directly to the UAN production section.

The used aqueous treatment liquid, containing water, ammonium nitrate and nitric acid, and resulting from scrubbing water vapor (i.e. the vapor stream from the concentration section) in the treatment section is preferably merged (e.g. combined) with the pH-controlled ammonium nitrate stream, or with the concentrated ammonium nitrate stream, i.e., downstream (for liquid) of the pH-control section. Preferably, said aqueous treatment liquid is recycled back to the inlet of the concentration section (which is preferably an evaporation section).

Recycling the treatment liquid back to the inlet of the concentration section provides as an advantage that the appropriate water balance is achieved for making UAN, i.e. sufficiently low water content of the UAN, with an advantageous relatively low temperature of the evaporation section and with advantageously a relatively high amount of treatment liquid in the treatment section.

Generally, in the event that pH control results in adjusting the pH, such adjustment can be beneficial be to prevent the aforementioned acid build-up, without being limited to that purpose. Accordingly, such pH adjustment involves the addition of base. With a view to the production of UAN, the base will typically be ammonia.

As a general preference, the pH control involves adjusting the pH, in particular increasing the pH by adding a base, in particular by adding ammonia. Adding ammonia as pH adjustment provides the above-discussed advantage regarding thermal stability of the ammonium nitrate, advantages during the evaporation, and contributes to the UAN solution having a desired pH close to 7.

Hence, in a preferred embodiment, the pH control involves adding a base, more preferably ammonia, even more preferably aqueous ammonia solution. Preferably, the pH-control step involves introducing an aqueous ammonia solution, preferably a dilute aqueous ammonia solution comprising ammonia, in an amount of 5 wt. % or less ammonia. Preferably the pH control section is configured for such a step. Preferably the pH control section comprises an inlet for introducing such solution, and a mixing zone (which may include a static mixer and/or active mixer and is e.g. a tie-in point). The system of the invention, as said, preferably is suitable for the conversion of any by-product or waste stream comprising ammonium nitrate, in particular aqueous ammonium nitrate. This generally relates to relatively small streams, or in any event aqueous ammonium nitrate streams of relatively low ammonium nitrate concentrations. In an example embodiment, the aqueous ammonium nitrate stream comprises, typically 5-20 wt. % of ammonium nitrate, more typically 5-15 wt. % of ammonium nitrate, and water, e.g. at least 80 wt. % water. The aqueous ammonium nitrate stream typically also comprises nitric acid, e.g. 0.1-0.5 wt. % nitric acid relative to the total stream. The aqueous ammonium nitrate stream preferably less than 5 wt. % urea, preferably less than 1.0 wt. % urea, e.g. only traces of urea. Traces of urea in the off-gases can remain in processes with dust scrubbing of waste air from urea finishing upstream of a waste air stream upstream of acid scrubbing. These example compositions apply also to the inventive process.

Such aqueous ammonium nitrate streams may originate, e.g., from acid scrubbing with nitric acid of a waste air stream comprising NHfrom a urea finishing sections where a urea melt is solidified into a solid urea product using cooling air (e.g. a granulator or prilling tower); other sources are also possible.

The entrained nitrogen, notably ammonia nitrate and nitric acid, in the water vapors resulting from subjecting such small aqueous ammonium nitrate streams to evaporation, in particular by heating, will have a correspondingly low concentration in the water vapors. The impact hereof, though, can be substantial. Even a small increase in the nitric acid content has a large impact on pH, and if small amounts or nitric acid continue to be processed, this can result an undesirable acid build-up. Moreover, the impact on safety is substantial, taking into account the aforementioned safety hazards resulting from a decreased decomposition temperature of ammonium nitrate. In an absolute sense, however, the amounts of nitric acid to be neutralized are low, in particular the amounts of nitric acid to be neutralized in the pH control section are small, at least relative to the total aqueous ammonium nitrate stream. As mentioned below, this brings about an additional process challenge

Thus, generally the amount of ammonia to be added to adjust the pH of the aqueous ammonium nitrate stream in the pH control section is small. For example, 2.3 kg/h of ammonia (i.e. 2.3 kg/h NH) would need to be added to a 2000 kg/h stream of 10 wt. % aqueous ammonium nitrate that contains 8 kg/h of nitric acid.

With such relatively low amounts of base needed, the flow rate of the base added upon pH-adjustment, is too low for allowing it to be regulated by a standard industrial valve. Whilst this can be solved by applying a small-scale laboratory-type valve, this has drawbacks. E.g., the valve needs to be placed in a specially designed cabinet to be protected against harsh environment. Also, the valve needs to be positioned very close to the line carrying the aqueous ammonium nitrate, which in practice is more difficult to accomplish in the case of a non-industrial type, typically small, valve. In the system of the invention, it is therefore desired to apply a regular, industrial-type valve, e.g. an at least 1 inch valve.

Preferably, the pH-control section is configured for introducing an aqueous ammonia solution, preferably a dilute solution comprising ammonia in an amount of at most 5 wt. % of ammonia, preferably between 0.5 and 5 wt. % of ammonia, most preferably 0.5 to 2 wt. %, into the aqueous ammonium nitrate stream. This provides the advantage, generally, that the aqueous ammonia solution has a relatively larger volume and can be more easily combined, in particular mixed, with the aqueous ammonium nitrate stream, to ensure optimum and homogeneous neutralization of the unreacted nitric acid in the aqueous ammonia solution, e.g. with a regular, industrial-type valve.

With reference to the pH control section as discussed above, it will be understood that the pH control is for instance implemented as a feedback control system or a feed forward system. In a feed forward system, process data and mass balance can be used to determine the pH and adjust it, as necessary based on the determined pH. Dilute ammonia solution is then preferably added downstream of the pH determination location. In a feedback system, pH can be measured downstream of the pH control section and feedback the info to the control section to adjust the pH as necessary. Here, the introduction of dilute ammonia is in fact upstream of the location of pH determination.

The system comprises a production section configured to mix concentrated ammonium nitrate and liquid urea so as to produce UAN, i.e. a UAN production section. The UAN production section is for example connected with the acidic ammonia scrubber of the chemical processing unit and pH control section as only sources of ammonium nitrate in the produced UAN.

In another aspect, the invention provides a process for the production of urea ammonium nitrate (UAN). The process is for example, but not exclusively, carried out in the system of the invention.

As a starting material to this process, an acid aqueous ammonium nitrate stream is provided, that results from subjecting an ammonia-containing off-gas, that preferably contains air, to scrubbing with nitric acid. The acid aqueous ammonium nitrate stream can also be referred to as a used scrubbing liquid and corresponds to the first used scrub liquid in.

The process can be described alternatively as a process for converting an acid aqueous ammonium nitrate stream that results from subjecting an ammonia-containing off-gas to scrubbing with nitric acid, into UAN. The process preferably comprises the step of scrubbing the ammonia-containing off-gas with nitric acid; the process then can alternatively be described as a process for acid scrubbing of an ammonia-containing off-gas stream using nitric acid scrub liquid with the production of UAN from the acidic ammonium nitrate solution resulting from said acid scrubbing.

In the scrubbing with nitric acid, the resulting cleaned off-gas and the acid aqueous ammonium nitrate stream are provided as separate streams. Hence, the acid scrubber has an outlet for vapor (cleaned off-gas) and a separate liquid outlet for acid aqueous ammonium nitrate stream. The acidic scrubber is configured for gas/liquid separation, e.g. by the gas outlet being at an upper part and the liquid outlet being at a lower part. The acid scrubbing is based on the counter-current contact of gas and nitric-acid containing liquid, typically with gas flowing up and liquid flowing down, for instance with off-gas containing air and NH.

This acid aqueous ammonium nitrate stream is distinct from the used treatment liquid from the treatment section; these are two different streams and having a different composition.

The process of the invention comprises obtaining the acid aqueous ammonium nitrate stream resulting as a used scrubbing liquid resulting from subjecting an ammonia-containing off-gas to contact with nitric acid in a scrubber to form ammonium nitrate.

The process in an embodiment comprises providing an ammonia-containing off-gas and scrubbing the off-gas with nitric acid in a scrubber thereby forming the acid aqueous ammonium nitrate stream, and a cleaned gas stream, in particular in an acid scrubber. The acid scrubber is for example operated at a pressure below 1.5 bar absolute, for instance at slight under pressure (0.8-1.0 bar absolute).

The process in an embodiment comprises subjecting a urea melt to solidification in a finishing section, e.g. a granulator or prilling tower, using cooling air to provide a solid urea product and a waste air stream comprising urea dust and NHand supplying the waste air stream, preferably after dust scrubbing, as at least part of said off-gas processed in said scrubber.

The process in a preferred embodiment comprises subjecting the waste air stream to dust scrubbing to remove at least part, e.g. at least 90 wt. %, of the urea dust from it, and supplying the waste air stream from said dust scrubber to said acid scrubber. The dust scrubbing uses, e.g. a circulating urea solution with aqueous make-up scrub liquid. Hence, preferably, the process comprises dust scrubbing of the waste air stream in a dust scrubber to remove urea dust, and acid scrubbing of the dust-scrubbed waste air stream in a separate acid scrubber with nitric acid to give the acid aqueous ammonium nitrate stream with a low urea content. In the invention, urea solution is added in the UAN production step. Thereby, judiciously the risk of urea degradation by nitric acid is reduced compared to processes wherein combined dust and acid scrubbing is used wherein the used scrub liquid contains urea, ammonium salt, and unreacted acid.

The acid aqueous ammonium nitrate stream comprises, for example, 5 to 50 wt. % ammonium nitrate, e.g. 5-20 wt. % of ammonium nitrate, preferably 5-15 wt. % of ammonium nitrate, and water, e.g. at least 80 wt. % water; and preferably less than 5 wt. % urea, or even less than 1.0 wt. % urea, such as only traces urea. The acid aqueous ammonium nitrate stream also comprises nitric acid and typically also comprises nitric acid, e.g. 0.1-0.5 wt. % nitric acid relative to the total stream. This composition applies e.g. at the inlet of the pH control step.

The pH of the acid aqueous ammonium nitrate stream is e.g. less than 3.0, or less than 2 or less than 1.5. Acid scrub at low pH advantageously may provide for lower gaseous NHs emissions from the waste air stream.